The present invention relates to a dosing device for introducing a liquid medium into an exhaust-gas stream of an internal combustion engine of a motor vehicle.
For internal combustion engines, adherence to limit values for pollutant emissions in the exhaust gas is a legal requirement. In the case of a diesel vehicle in particular, nitrogen oxide reduction is imperatively necessary. One possibility for nitrogen oxide reduction is, for example, the known method of selective catalytic reduction (SCR). In said system, a liquid reducing agent, for example a urea-water solution, is introduced into the exhaust-gas stream in the exhaust pipe. With the hot exhaust gas, ammonia gas is generated from the urea-water solution, by means of which ammonia gas the nitrogen oxide, which is harmful to health, is reduced to form non-hazardous water and nitrogen.
DE 10 2010 28 866 A1 has disclosed a device for introducing the urea-water solution into the exhaust-gas stream. The urea-water solution is pressurized by means of an electrically driven pump. The injection itself is controlled by way of the electric actuation of a dosing valve. Owing to the electric actuator means, the dosing valve must generally be cooled. In the case of said system, a gas mixer is generally necessary in order to ensure a homogenous distribution of the urea-water solution.
A dosing device known from DE 10 2011 078 850 A1, which constitutes a subsequent publication, is controlled by pressure waves. A valve needle of a dosing valve is held closed by means of a spring holder and automatically opens above a certain hydraulic pressure. The injection is thus controlled by way of the pressure wave that is generated by means of a pump. The metering accuracy of said system is not particularly high because the injection quantity is dependent on a profile of the pressure wave with respect to time, and said pressure wave form is influenced by external influences.